1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and more particularly, to a zoom lens suitable for use in a broadcasting TV camera, a video camera, a digital still camera and a silver-halide film camera.
2. Description of the Related Art
In recent years, a zoom lens having a high zoom ratio, a wide angle of field and a high optical performance is desired for use in an image pickup apparatus such as a TV camera, a silver-halide film camera, a digital camera or a video camera.
A positive lead type four-unit zoom lens has been known as the zoom lens having a high zoom ratio and a wide angle of field. In the four-unit zoom lens, four lens units are provided in total, and one of the lens units located on the most object side has a positive refractive power.
For example, there has been known a four-unit zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power for magnification-varying, a third lens unit for correcting image plane variation, and a fourth lens unit having a positive refractive power (Japanese Patent Application Laid-Open No. 2004-264457, Japanese Patent Application Laid-Open No. 2008-197533 and Japanese Patent Application Laid-Open No. 2010-091788).
Japanese Patent Application Laid-Open No. 2004-264457 discloses a zoom lens having an angle of field of 60.93° at a wide-angle end and having a zoom magnification of about 96. Japanese Patent Application Laid-Open No. 2008-197533 discloses a zoom lens having an angle of field of 69.02° at the wide-angle end and having a zoom magnification of about 106. Japanese Patent Application Laid-Open No. 2010-091788 discloses a zoom lens having an angle of field of 62.86° at the wide-angle end and having a zoom magnification of about 120.
When a high zoom ratio (high magnification) is to be achieved in the above-mentioned four-unit zoom lenses, it is necessary to secure a long movable range for the second lens unit for magnification-varying, and thus the lens diameter of the first lens unit disposed on the most object side tends to be increased. Further, when a wide angle of field is to be achieved, it is necessary to secure a wide incident angle for beams to be received by the first lens unit, and thus the lens diameter of the first lens unit tends to be increased as well. In order to prevent an increase in the lens diameter of the first lens unit while achieving both a wide angle of field and a high magnification, it is important to reduce the size of the second lens unit for magnification-varying, and to shift a principal point position of the second lens unit toward the object side, which is a reference position for incident beams.
In the four-unit zoom lenses having the above-mentioned structure, in order to achieve a high zoom ratio and a wide angle of field, and also to achieve high optical performance over the entire zoom range while achieving a size reduction for the entire lens system, it is important to appropriately set the refractive power of the first lens unit, the lens structure of the second lens unit for magnification-varying, and the like.
If those settings are inappropriate, attaining a compact zoom lens having a high zoom ratio and a wide angle of field becomes difficult.